1. Field of the Invention
The present invention concerns an air/fuel ratio control apparatus for an internal combustion engine, which is located in the internal combustion engine possessing a discharge prevention mechanism for fuel vapors, and appropriately controls the air/fuel ratio of the air-fuel mixture. In particular, it concerns realization of the appropriate control mechanism structure, adopting a system in which a linear air/fuel ratio sensor is employed to carry out feedback control of the air/fuel ratio.
2. Description of Related Art
As is known, a discharge prevention mechanism for fuel vapor involves a mechanism for storage in a canister of fuel vapor generated from a fuel tank, and passage of the stored fuel vapor through a purge passage for discharge to an air intake system of an internal combustion engine in order to prevent the fuel vapor from being discharged to the exterior.
Moreover, regarding discharge of the fuel vapors to the engine air intake system, it is well known that there is usually passage through a flow volume control valve known as a purge valve disposed in a purge passageway in order to control the fuel vapor flow volume in the passageway, that is, the purge flow volume.
However, although the purge flow volume is generally adjusted as a volume in proportion to the engine air intake volume, because the flow volume is adjusted and controlled separately from normal fuel injection control, during the period that purge is taking place, discrepancies can readily occur in the air/fuel ratio that has been set according to the running conditions of the engine.
Conventionally, there has been proposal of an air/fuel ratio control apparatus such as disclosed in Japanese Non-Examined Patent Publication No. Sho 63-41632 in order to deal with such discrepancies in the air/fuel ratio.
In other words, with the device described in the above patent publication, a prerequisite is a system for feedback control of the air/fuel ratio based on an output of an air/fuel ratio sensor (oxygen density sensor: O.sub.2 sensor) installed in an engine exhaust system.
Purge control can be roughly divided into three main steps:
(1) learning a deviation of a feedback value (air/fuel ratio compensation coefficient FAF) based on whether there is purge or not; PA1 (2) computation of a fuel compensation amount according to the purge based on the learning value and purge flow volume; and PA1 (3) compensation of a basic fuel injection volume based on the fuel compensation amount that is computed. PA1 it is a fluctuation in the feedback value (air/fuel ratio compensation coefficient FAF) based on purge; or PA1 it is a fluctuation in the feedback value (air/fuel ratio compensation coefficient FAF) based on transient running, gear shift changing and other factors. PA1 Compensation of the compensation coefficient when the change amount of the fuel vapor flow volume ratio (purge ratio) exceeds a set value; or PA1 Compensation of the compensation coefficient when the change rate of the compensation coefficient due to change in the fuel vapor flow volume ratio (purge ratio) exceeds a set value. PA1 Compensation of the compensation coefficient at a value in which the computed compensation value deltaFAFi is appropriately averaged.
Carrying out such purge control prevents the discrepancies in the air/fuel ratio resulting from purge.
In order to maintain such purge controllability in all engine operational ranges, there is naturally a need for controllability with a high level of accuracy regarding the purge flow volume.
Actually, however, due to such factors as insufficient linearity of the flow characteristics of the purge valve itself as well as tolerance and computation deviations for the purge flow volume, there is considerable scattering of the learning value for deviation in the feedback value (air/fuel ratio compensation coefficient FAF) based on whether there is the purge or not, as well as the purge fuel compensation amount based on this.
It can also be understood here that, because the purge flow volume is determined by the pressure difference before and after the purge valve and by the valve aperture, accurate computation with an actual vehicle is difficult.
Also, in order to obtain high controllability regarding the purge flow volume, there is naturally a need in the purge valve for an expensive control valve and extensive control logic, thus leading to a major increase in production costs.
On the other hand, although the conventional apparatus features feedback control of the air/fuel ratio based on the output from the air/fuel ratio sensor, it presupposes use of the O.sub.2 sensor as the air/fuel ratio sensor. In cases employing a linear air/fuel ratio sensor of the type that has been frequently used in recent years, the following further problems result.
Regarding a linear air/fuel ratio sensor that detects linearly the air/fuel ratio of the air-fuel mixture from the oxygen density in the exhaust gas, the feedback response is very high compared with the O.sub.2 sensor, so that it has become possible to also accurately detect deviations in the air/fuel ratio even in short cycles that could not be detected with feedback from the O.sub.2 sensor. As a result, even in learning the feedback value (air/fuel ratio compensation coefficient FAF) depending on whether there is purge or not, it is not possible to distinguish whether:
As a result, the reliability of the learning value itself is negatively effected.
If the reliability of the learning value decreases with the conventional apparatus, there is also a negative effect on controllability concerning the air/fuel ratio, which in turn can bring about worsening of emissions and a reduction in drivability.